When a high-speed signal is travelling through a long transmission channel, the signal will be degraded due to the channel's bandwidth limitations. There are two main mechanisms that attenuate high-frequency signals in transmission lines: the skin effect and dielectric absorption. The skin effect is the tendency of high-frequency signals to travel on the surface of the material, causing an increase in inductive reactance. Dielectric absorption arises from the tendency of high-frequency signals to excite molecules of insulating material adjacent to the signal line. As a result, energy is absorbed and signal strength is reduced.
The frequency response of a transmission channel can be modeled as a low-pass transfer function, as shown in equation:Atten=d*((K sin*sqrt(f))+(Kdielectric*f))where d is the length of the transmission line and f is the frequency of operation. Kskin is a constant that is inversely proportional to the skin depth of the conductor and the width of the transmission line. Kdielectric is also a constant that is proportional to the material dissipation factor and square root of the real part of the dielectric constant.
In digital signaling the highest frequencies comprise signal transitions such as the rising and falling edges of a binary data symbol. When the data rate is sufficiently high, the binary data transitions cannot complete transition within a symbol interval, and symbol energy will spread into the adjacent symbols. This spreading of symbol energy is defined as inter-symbol interference (ISI). To reduce the effect of ISI, the signal amplitude is boosted during transition, which amounts to a high-frequency boost in the frequency domain. This boost is referred to as high-frequency emphasis. High-frequency emphasis of the signal may also be referred to as pre-emphasis and such terms are used interchangeably herein. A pre-emphasis circuit acts as an equalizer that boosts the high-frequency component and compensates for attenuation of the signal as the signal is transmitted through the channel.
The majority of transmit driver circuits with pre-emphasis utilize current mode logic (CML) based topologies. In current mode line drivers, the output state has two linear 50 ohm resistors to the power supply, two differential pairs, a main tail current source and an auxiliary tail current source. The auxiliary tail current emphasizes the signal when enabled. Both tail currents can be programmable depending on the steady state output voltage swing and the amount of voltage boost required.
While the current mode logic topology may offer a good return loss performance because the internal linear resistors can be accurately matched to the external load resistance, it is inefficient from a power usage standpoint. In order to perform pre-emphasis in a current mode logic based topology, the main and auxiliary tail currents must be four times the load current in order to sustain the proper steady and emphasized voltage levels of the output signal. As a result, when the maximum differential voltage increases due to an increase in boost, the output stage current will increase by a factor of four. The pre-driver circuit also consumes a significant amount of power as it needs to drive the gate to the source capacitance of the differential pairs of the output stage.
One or more embodiments of the present invention may address one or more of the above issues.